See-saw and supersymmetry or exact R-parity

We show how the renormalizable see-saw mechanism in the context of supersym metry and spontaneously broken B - L symmetry implies exact R-parity at all energies. We argue that supersymmetry plays an important role in providing a ''canonical'' form for the see-saw, in particular in grand unified theor ies that solve the doublet-triplet splitting problem via the Dimopoulos-Wil czek mechanism. (C) 1999 Published by Elsevier Science B.V. All rights rese rved.